The present invention relates to a device enabling a dynamic measurement of the distance between the facing faces of the rotor and the stator of a rotary machine to be made by a capacitive method. The invention is particularly applicable to the dynamic measurement of the radial clearance existing between the tips of the blades of a turbo-machine wheel and the wall of the casing which surrounds the wheel.
2. Summary of the Prior Art
The clearance existing between the facing faces of the rotor and the stator of a rotary machine is an important parameter which has an appreciable influence upon the performance of the machine, whatever the type of rotary machine involved, and this applies particularly to the clearances between the tips of the rotor blades and the surrounding casing in compressors and turbines. As these clearances may vary substantially with thermal and mechanical changes, it is of the utmost importance to engine designers to be conversant with the changes in these clearances over the whole of the circumference of the interface between the rotor and stator in an actual machine operating at a constant speed or at a transitional speed. To carry out dynamic measurement of this clearance, it is known to use electrical, optical or radiofrequency based proximity sensors.
The invention relates to a device of the type which carries out the measurement using a capacitive electrically based method involving measuring the load of a capacitor formed by a conductive surface of a sensor fixed in the stator opposite the rotor and a conductive surface of the rotor which is brought opposite the conductive surface of the stator in the course of the rotation of the rotor relative to the stator.
The characteristics of a device for measuring by this capacitive method are dependent upon the nature of the polarization and the nature of the conditioning of the capacitive sensor.
Certain known measuring devices utilize a capacitive sensor polarized by an alternating voltage and conditioned either by an oscillator or by a load amplifier. In these devices, the technology of the capacitive sensor and the connecting cable between the sensor and the measuring chain is generally triaxial.
Other known measuring devices utilize a capacitive sensor polarized by a direct voltage and conditioned by a voltage amplifier, the technology of the sensor and the connecting cable between the sensor and the measuring chain in this case being generally coaxial.
In all these known devices, the sensor and the connecting cable have residual capacitances which affect the calibration of the measuring chain. Calibration must therefore be carried out every time there is a change of sensor or connecting cable.
Moreover, all these known devices require test calibration of the measuring chain on site, which does not enable the manufacturer to carry out calibration with a view to reproducible series usage.
Finally, for all these known devices, the measurement results are influenced by the temperature of the sensor and of the connecting cable.